1. Field of the Invention
The present invention relates to a recording material for electro-coagulation printing and a method for electro-coagulation printing on the same. More particularly, the present invention relates to a recording material usable for forming colored images having a high clarity by an electro-coagulation printing procedure, and an electro-coagulation printing method for forming colored images having a high clarity on the recording material.
2. Description of the Related Art
A basic principle of an electro-coagulation printing method (which may be referred to as an elcography) is disclosed in U.S. Pat. No. 3,892,645 and Japanese PCT Application Publication No. 4-504,688 (PCT International Publication WO 90/11897). This printing system is a non-printing plate printing system and uses an aqueous ink having such a property that it is gelled with an electric charge.
In elcography, the specific aqueous ink as mentioned above is applied between a positive cylindrical electrode and a negative electrode, and an electric differential potential corresponding to an imagewise signal is applied between the positive and negative electrodes to generate metal ions, to coagulate portions of the aqueous ink layer formed on the cylindrical electrode by the metal ions, and to cause the coagulated ink colloid to adhere to the surface of the positive cylindrical electrode. By controlling the amount and location of the coagulated ink colloid, a desired colored ink image can be formed on the positive cylindrical electrode surface, and a colored image formed from the electro-coagulated ink can be transferred onto a surface of a recording material under pressure. The constitution of a printing machine for the elcographic printing method will be explained later by referring to an attached drawing.
The elcographic printing method is characterized by not using a printing plate. A non-printing plate printing method is advantageous in that no procedure for providing a printing plate is necessary, desired images can be easily printed on a recording material surface in accordance with electric signals corresponding to the desired images, and thus various prints recording images different from each other can be easily prepared for various customers. Especially, the elcographic printing method is advantageous for various lots of prints each in a small number and different from one another.
Also, the elcographic printing method is advantageous in that since the amount of gelled ink, which corresponds to the color density of the images, is proportional to the amount of the electric charge (pulse), ink images having a fine and sharp color tone can be recorded.
Since in the elcography, no dots or halftone dots are used, the color tone of the images can be controlled by regulating the thickness of the ink layers, as in gravure printing. The thickness of the ink layers is proportional to the electric potential-applying time in accordance with the electric signals, and thus the color tone of the images can be accurately controlled. Therefore, the elcographic printing method is suitable for recording colored images.
Conventional printing paper sheets have a satisfactory absorption of oil-based inks, but are poor in absorption and transferring property of the aqueous inks usable for the elcographic printing. Also, when the elcography is applied to multi-color printing, the water component in a first colored aqueous ink applied onto the recording material surface must be quickly absorbed by the recording material, before the second colored ink is applied onto the first colored ink images. If the second colored aqueous ink is applied onto the first colored aqueous ink images before the water component of the first colored aqueous ink images is fully absorbed by the recording material, the second colored aqueous ink is not fully transferred to the recording material and, thus, the desired colored images cannot be obtained.
As mentioned above, the elcographic printing method is a very advantageous printing method. However, this method is disadvantageous in that when conventional paper sheets or printing sheets are used, the advantages of elcography cannot be fully obtained. For example, when a conventional printing paper sheet is used, prints having excellent printing qualities both in high color density tone and low color density tone cannot be obtained.
When a conventional art paper sheet is used, the resultant printed images, in a low color density region in which the amount of the coagulated ink colloid is small, exhibit a sufficient color density and thus the resultant prints are satisfactory. However, in a high color density region in which the amount of the coagulated ink colloid is large, the resultant colored ink images exhibit a poor transferring property. Also, when conventional newspaper sheets or form-printing paper sheets are used, the elcographic printing in the high color density region, in which the amount of the coagulated ink colloid is large, can be satisfactory effected. However, in the low color density region in which the amount of the coagulated ink colloid is small, the elcographic printing is disadvantageous in that non-printed white spots and reduced color density spots are formed.